1. Field of the Invention
The present invention relates to an apparatus and method of an electronic ballast to ignite a high intensity discharge (HID) lamp. In particular, the present invention is directed to operating a good HID lamp with a square waveform voltage and current, and to shutdown (e.g., turn OFF) a ballast associated with the HID lamp so as to terminate electrical output power to the HID lamp when an end-of-life (EOL) condition of the HID lamp is detected, and to then re-engage the ballast to re-supply electrical power to the HID lamp after a predetermined period of time (such as, for example, a few minutes) passes from the shutdown. During operation of the HID lamp, a predetermined lamp voltage, such as, but not limited to, for example, a low frequency, square wave lamp voltage, is continuously monitored. The level of a difference between a lamp re-ignition voltage and a normal lamp voltage is calculated every lamp voltage half cycle and compared to a series of pre-defined, graduated threshold levels of the difference of the lamp re-ignition voltage and the normal lamp voltage. Whenever the lamp re-ignition voltage to normal lamp voltage difference is higher than a threshold level of the lamp re-ignition voltage to normal lamp voltage difference, it is recorded. The HID lamp is determined to have reached its EOL condition when the number of records in a rolling measurement window becomes greater than a pre-defined percentage. At that time, the ballast is turned OFF to discontinue (terminate) the electrical output power supply to the HID lamp.
According to a preferred embodiment of the present invention, a frequency of the low frequency, square wave voltage and current is generally above 10 Hz and below 1000 Hz. However, this upper range and/or lower range may be varied without departing from the spirit and scope of the invention.
2. Background
As technology evolves, lamp manufacturers are developing smaller and more compact high intensity discharge lamps to satisfy the needs of end users. For example, some lamps are designed with a miniaturized fixture, in which a fixture protective lens is not required. When the lamp (e.g., an arc tube lamp) leaks, a gas, such as, but not limited to, for example, an argon gas from the lamp or when an outer envelope leaks, a nitrogen/oxygen gas mixture, etc. is released in a space between the arc tube lamp and an outer jacket, which is typically a vacuum. A resulting outer jacket gas pressure of the, for example, argon gas is a function of a numerical ratio of an arc tube volume to an outer jacket inner volume. Because of a relatively large outer jacket inner volume compared to the arc tube lamp volume, the resulting outer jacket gas pressure (e.g. argon) is significant higher than the vacuum. As a result, electrically insulating properties of the vacuum are compromised, and a breakdown voltage of the outer jacket is substantially reduced, such that stem lead arcing can occur when an ignition voltage is applied to the lamp.